Collections were an interesting idea when introduced, but no desktop sync and poor marketing (why exactly should they be used instead of bookmarks? why were they added at all?) discouraged from them many people. Years passed, but I haven't noticed any significant changes in this matter.
That's why I think it would be beneficial if we could learn more about what their users think. Why do use collections? Do you use them instead of bookmarks or along them? What makes them great and which parts require more work? How can they be improved?
Funnily enough I was browsing for places to visit on a trip I'm planning and noticed the Collections feature, so thought I'd use it. However, I've realised after reading your post that they don't sync. Judging from this article, it seems like it's an Android only feature, which is a shame. Ideally, they would be accessible on a desktop usimg Firefox Sync.
This is a great topic, @Ponda, thanks for kicking off this discussion! Interested to hear more feedback. In the meantime, I'll drop some threads/ideas related to Collections: